Exposing structural material to an aggressive environment under steady or cyclic stress can cause damage in the form of cracking. This result is generally referred to as stress corrosion cracking ("SCC") or corrosion fatigue. For example, metallic alloys used as structural members often operate under sustained or cyclic stress and in high-temperature, oxygenated water, such as that found in boiling water reactors (BWRs), where temperatures and pressures exceed 280.degree. C. and 1000 psi, respectively. The BWR environment may also be radioactive and provide only limited space to accommodate the placement of a sensor to monitor SCC.
Damage in the form of SCC, or other stress/environment induced cracking, is of greater concern to the nuclear industry. The problems which industry faces in attempting to predict the onset of, or susceptibility of particular structural components to SCC under specific operating conditions are set out in U.S. Pat. No. 4,677,855 to Coffin, Jr. et al. and U.S. Pat. No. 4,923,708 to Solomon et al., the disclosures of which are incorporated by reference herein.
Methods for measuring crack growth which require test specimens to be removed from their testing environment have been disclosed over the years. These methods use a variety of monitoring systems including visual and electrical potential (voltage) drop measurements. The electrical potential drop method improved the art by enabling mathematical models to be generated which were effective in monitoring the rate of crack propagation in a specimen. These models are described in detail in the Coffin and Solomon patents.
Material testing for SCC is often carried out using a specimen constructed in a double cantilever beam (DCB) geometry, which has a machined slot at its root notch. The bifurcate specimen is then loaded to force apart the beams of the DCB, e.g., using an electrically nonconductive wedge, thereby inducing crack growth at the machined slot. The rate of crack growth may be monitored and used to assess the aggressiveness of the specimen's environment.
Preferably, the intensity of the applied stress, termed the "stress intensity factor", remains constant at the leading edge of the crack. The rate of crack growth would then be expected to be constant, given an unvarying environment. However, the wedge loading approach results in a nonconstant load as the DCB material creeps and as the DCB material's compliance increases due to a growing crack length. A decreasing stress intensity causes the crack growth rate to slow. Eventually, the stress intensity factor may decrease below the threshold required for SCC. Crack growth will then stop and the sensor will no longer function as an SCC monitor unless additional load is introduced.
One method for applying a constant load to a specimen is to hang an appropriately sized weight from the end of one of the beams while the other beam is captively retained. Here, unlike the wedge loading arrangement, an increase in specimen compliance, resulting from increased crack growth, increases the stress intensity factor. This is not desired since a constant, rather than increasing, stress intensity factor isolates changes in crack growth as being due to changes in the specimen environment.
Generally large testing machines and devices capable of applying loads to DCB specimens have long been known and are in common use. Such machines may involve arms that interlock with the beam components of the DCB specimen and, when energized, move apart, widening the specimen slot. An example of an apparatus with utilizes this method of loading is disclosed in U.S. Pat. No. 4,481,826 to Ingraffea, which discloses the use of expandable jaws driven apart by a threaded knob.
Alternatively, a loading machine may consist of an inflatable pressure bag which, when installed between the beams of a DCB specimen and inflated, tends to widen the specimen slot. This type of apparatus is disclosed in U.S. Pat. No. 4,075,884 to Barker.
However, neither device disclosed by Ingraffea or Barker provides for monitoring the load actually applied to the specimen beams. In addition, these configurations require too much space to be practically employed in BWR applications. Finally, the pressurized bag disclosed by Barker has a nearly infinite compliance and is therefore incapable of being pre-tensioned. Since the specimen loading device of Barker has little inherent spring modulus, it must rely wholly on external fluid pressure to exert any load on the arms of a test specimen.
Thus, there has been a need for a method for applying a constant stress intensity to a remotely mounted DCB specimen. With a constant stress applied, the rate of crack growth can be correlated with the effects of adjusting the specimen's environment. SUMMARY OF THE INVENTION
The present invention is directed to an improved apparatus and method for controlling the crack growth rate within a DCB type test specimen. This specimen is provided with a pre-formed crack at the root of the notch between its two beams. The DCB specimen is then exposed to an aggressive environment while an adjustable load is applied to its beams sufficient to induce the pre-formed crack to grow. Crack length is monitored, such as by the voltage drop method, and the rate of crack growth is determined.
The DCB geometry may vary in length and cross section depending upon the physical restrictions of a particular application. In the extreme, the invention may also be applied to a short stiff fracture specimen such as a compact tension ("CT") specimen. CT specimens are described in detail in ASTM Standard E 399-83, entitled "Standard Test Method for Plane-Strain Fracture Toughness of Metallic Materials" the disclosure of which is incorporated by reference herein.
In accordance with the invention, the arms of a DCB type test specimen are fitted with a pressure-actuated bellows for inducing a load and a sensing assembly for providing load control feedback. In particular, the load control feedback sensing assembly comprises a proximity device for determining the resulting crack opening displacement. It is also envisioned that a small, compressive load cell may be employed to provide load control feedback. Having measured the crack length and knowing the specimen compliance, i.e., the amount of deflection exhibited by a specimen when subjected to a given load, displacement of the specimen arms and the spring constant of a bellows-type loading mechanism, the stress intensity at the crack tip may be calculated. Pressure applied to the bellows loading mechanism may be adjusted, as necessary, to accommodate for the temperature and/or stress-induced material creep or the reduced loading due to DCB crack growth, thereby allowing a constant stress intensity to be maintained, if desired. Furthermore, the specimen may be preloaded by mechanically compressing the bellows between the specimen arms, thereby utilizing the inherent spring constant of the bellows to provide partial loading to the specimen. The bellows spring constant, as used herein, equals the amount of additional force necessary to deflect the unrestrained, unpressurized bellows an additional unit distance.
During separation of the ends of the DCB sensor beams, the pressurized bellows expands by an amount equal to the relative displacement of the beam ends. As the bellows expands, the spring load exerted by the bellows (which is independent of the gas pressure in the bellows) decreases. Accordingly, to calculate the amount of force being exerted by the pressurized bellows, the decrease in spring load must be taken into account. Thus, unlike the flexible bag disclosed in Barker, the bellows-loaded DCB crack growth sensor of the present invention requires a means for accurately determining the relative displacement of the beam ends, which information enables computation of the variation in spring force due to the bellows.
The measurement and control technique of the invention may be used as a water chemistry check monitor or to assist in determining or estimating the damage caused by aggressive environments on structural components exposed to stress. Sensor crack growth may then be monitored as the chemistry of the environment is altered to determine the effect changing various environmental factors has on SCC.
Alternatively, the apparatus and method of the invention may be employed in materials research to identify and predict the relative susceptibility of various material compositions to SCC under simulated environmental conditions for both BWR and non-BWR applications.